This invention relates generally to electronic musical instruments and more particularly to techniques in frequency deviation and in generating tremolo effects. More specifically, the invention relates to a new and advanced frequency-deviation method wherein the frequencies of specific signals such as voice signals or musical tone signals are caused to deviate slightly up and down and to a new method and apparatus for utilizing this frequency-deviation method to generate tremolo-effect signals.
One example of a method known heretofore whereby the frequency of a specific signal as, for example, a musical-tone signal, is caused to deviate slightly is that wherein (as indicated in FIG. 1 described hereinafter) a carrier signal f.sub.01 (e.g., 50 kHz) from an oscillator O.sub.1 of a frequency amply higher than that of the musical-tone signal (the signal usually comprises a plurality of frequency components thus constituting a frequency spectrum band, but for convenience in explanation in this specification, the spectrum band is represented by only a single frequency f) is subjected to balanced modulation by the musical-tone signal f in a balanced modulator M. The resulting double-size band (or both-side band) signal having frequency components of f.sub.01 - f and f.sub.01 + f is demodulated in a demodulator D through the use of a signal f.sub.02 of a frequency slightly higher (or lower) by .DELTA.f(for example, from 1 to 10Hz) than the above mentioned signal f.sub.01, the signal f.sub.02 being obtained from a second oscillator O.sub.2, whereupon a signal f-.DELTA.f and a signal f+.DELTA.f are produced as the demodulated output.
However, this known method requires a balanced modulator, a demodulator, and two oscillators, whereby the organization is complicated. Moreover, in the case where .DELTA.f is made small, an extremely high degree of frequency stability of both oscillators becomes necessary. For this purpose, oscillators of high precision are required and entail high cost.
Furthermore, the only tremolo devices of electronic musical instruments which have heretofore been reduced to practice are those in which musical-tone signals are subjected to amplitude modulation with a specific period and waveform, and in which electrical signals are converted into audible signals at a loudspeaker, during which operation, by a method such as rotating the loudspeaker with a specific period, a tremolo effect or a chorus effect (the result of modulated frequencies of from 5 to 10 Hz being ordinarily referred to as a tremolo effect, and that of modulated frequencies of from 0.5 to 2 Hz being ordinarily referred to as a chorus effect in electronic musical instruments) is obtained.
In the former method, however, since only amplitude modulation is resorted to, the frequency of the musical tone does not vary, and the tone is monotonous, whereby the resulting effect is extremely weak. In the latter method, a Doppler effect is produced by the rotation of the loudspeaker, and there are variations in the frequency, phase, and directivity of the musical-tone signal. Furthermore, variation in the signal level (amplitude modulation) is added and imparts a swelling to the sound, whereby the result is highly effective. On the other hand, however, the device for rotating the loudspeaker and the driving mechanism therefor is unavoidably complicated and expensive, and, furthermore, objectionable noises such as mechanical noises and the sound of rotating parts such as the loudspeaker traveling through the air are produced.